1. Field of the Invention
The present invention relates to liquid crystal display devices. More particularly it relates to liquid crystal display devices implenting in-plane switching (IPS) where an electric field to be applied to liquid crystals is generated in a plane parallel to a substrate.
2. Discussion of the Related Art
A liquid crystal display device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. Liquid crystal molecules have a definite orientational alignment as a result of their long, thin shapes. That alignment direction can be controlled by an applied electric field. In other words, as an applied electric field changes, so does the alignment of the liquid crystal molecules. Due to the optical anisotropy, the refraction of incident light depends on the alignment direction of the liquid crystal molecules. Thus, by properly controlling an applied electric field, a desired light image can be produced.
Of the different types of known liquid crystal displays (LCDs), active matrix LCDs (AM-LCDs), which have thin film transistors (TFTs) and pixel electrodes arranged in a matrix form, are the subject of significant research and development because of their high resolution and superiority in displaying moving images.
LCD devices have wide application in office automation (OA) equipment and video units because they are light and thin and have low power consumption characteristics. The typical liquid crystal display panel has an upper substrate, a lower substrate and a liquid crystal layer interposed therebetween. The upper substrate, commonly referred to as a color filter substrate, usually includes a common electrode and color filters. The lower substrate, commonly referred to as an array substrate, includes switching elements, such as thin film transistors and pixel electrodes.
As previously described, LCD device operation is based on the principle that the alignment direction of the liquid crystal molecules is dependent upon an electric field applied between the common electrode and the pixel electrode. Thus, the alignment direction of the liquid crystal molecules is controlled by the application of an electric field to the liquid crystal layer. When the alignment direction of the liquid crystal molecules is properly adjusted, incident light is refracted along the alignment direction to display image data. The liquid crystal molecules function as an optical modulation element having variable optical characteristics that depend upon polarity of the applied voltage.
In a conventional LCD device, since the pixel and common electrodes are positioned on the lower and upper substrates, respectively, the electric field induced between them is perpendicular to the lower and upper substrates. However, the conventional LCD devices having the longitudinal electric field have a drawback in that they have a very narrow viewing angle. In order to solve the problem of narrow viewing angle, in-plane switching liquid crystal display (IPS-LCD) devices have been proposed. The IPS-LCD devices typically include a lower substrate where a pixel electrode and a common electrode are disposed, an upper substrate having no electrode, and a liquid crystal interposed between the upper and lower substrates. A detailed explanation about operation modes of a typical IPS-LCD panel will be provided referring to FIGS. 1, 2A, and 2B.
FIG. 1 is a schematic cross-sectional view illustrating a concept of a related art IPS-LCD panel. As shown in FIG. 1, upper and lower substrates 10 and 20 are spaced apart from each other, and a liquid crystal layer 30 is interposed therebetween. The upper and lower substrates 10 and 20 are often referred to as an array substrate and a color filter substrate, respectively. On the lower substrate 20 are a common electrode 22 and a pixel electrode 24. The common and pixel electrodes 22 and 24 are aligned parallel to each other. On a surface of the upper substrate 10, a color filter layer (not shown) is commonly positioned between the pixel electrode 24 and the common electrode 22 of the lower substrate 20. A voltage applied across the common and pixel electrodes 22 and 24 produces an electric field 26 through the liquid crystal 32. The liquid crystal 32 has a positive dielectric anisotropy, and thus it aligns parallel to the electric field 26.
FIGS. 2A and 2B conceptually help illustrate the operation of a related art IPS-LCD device. When no electric field is produced by the common and pixel electrodes 22 and 24, i.e., off state, as shown in FIG. 2A, the longitudinal axes of the liquid crystal (LC) molecules 32 are parallel and form a definite angle with the common and pixel electrodes 22 and 24. For example, the longitudinal axes of the LC molecules 32 are arranged parallel with both the common and pixel electrodes 22 and 24.
On the contrary, when a voltage is applied to the common and pixel electrodes 22 and 24, i.e., on state, as shown in FIG. 2B, an in-plane electric field 26 that is parallel to the surface of the lower substrate 20 is produced because the common and pixel electrodes 22 and 24 are on the lower substrate 20. Accordingly, the LC molecules 32 are re-arranged to bring their longitudinal axes into coincidence with the electric field. However, the first LC molecules 32a positioned corresponding to (e.g., above) the common and pixel electrodes 22 and 24 do not change their orientation, while the second LC molecules 32b positioned between the common and pixel electrodes 22 and 24 are arranged perpendicular to the common and pixel electrodes 22 and 24. Therefore, the result is a wide viewing angle that ranges from about 80 to 85 degrees in up-and-down and left-and-right sides from a line vertical to the IPS-LCD panel, for example.
FIG. 3 is a plan view illustrating one pixel of an array substrate according to a related art IPS-LCD device. As shown, gate lines 40 are transversely arranged and data lines 52 are disposed substantially perpendicular to the gate lines 40. A common line 42 is also transversely arranged parallel with the gate line 40. The gate line 40, the common line 42 and a pair of the data lines 52 define a pixel region P on the array substrate. An island-shaped semiconductor layer 46 is positioned near the crossing of the gate and data lines 40 and 52, thereby forming a thin film transistor (TFT) T with a source electrode 48 and a drain electrode 50. A portion of the gate line 40 near the crossing acts as a gate electrode in the TFT T.
In each one pixel, three common electrodes 44 extend from the common line 42, and two of the common electrodes 44 are disposed next to the data lines 52, respectively. A capacitor electrode 54 is disposed over the common line 42, and connects pixel electrodes 56 that are disposed between the common electrodes 44 parallel with the data lines 52. One of the pixel electrodes 56 is electrically connected to the drain electrode 50 of the TFT T. The common line 42, the common electrodes 44 and the gate lines 40 are made of the same opaque material, which has low specific resistance, and maybe formed by the same process. The capacitor electrode 54 and a portion of the common line 42 form a storage capacitor Cst with a dielectric layer interposed therebetween.
In the IPS-LCD device shown in FIG. 3, it is very important that the common electrode 44 should be disposed between the data line 52 and the pixel electrode 56. That is, because an electrical field occurring in the data line 52 affects the pixel electrode 56, cross talk can occur between the data line 52 and the pixel electrode 56 if the pixel electrode 56 is disposed next to the data line 52. The electrical interference between the data line 52 and the pixel electrode 56 causes a decrease of the image quality in the IPS-LCD device.
As mentioned before, the TFT T includes the island-shaped semiconductor layer 46 and the source and drain electrodes 48 and 50 over the semiconductor layer 46. The TFT T also includes the portion of the gate line 40 as a gate electrode. The source electrode 48 extends from the data line 52 and the drain electrode 50 is spaced apart from the source electrode 48. The island-shaped semiconductor layer 46 can be disposed under the gate line 40 in order to protect the island-shaped semiconductor layer 46 from incident light. In the IPS-LCD device shown in FIG. 3, the thin film transistors of the up-and-down neighboring pixel are adjacent to each other and possess the same source electrode jointly. Thus, the source electrode extending from the data line 52 has a “T” shape to be jointly used for the up-and-down neighboring thin film transistors. Semiconductor pattern 47 in the crossing of the data line 52 and the gate lines 40 prevents a short between the data line 52 and each gate line 40. Likewise, the semiconductor pattern 47 in the crossing of the data line 52 and the common line 42 prevents a short between the data line 52 and the common electrode 44.
FIG. 4 is a cross-sectional view taken along line IV—IV of FIG. 3 and illustrates lower and upper substrates of the related art IPS-LCD device. As shown, lower and upper substrates 60 and 70 are spaced apart from and face to each other. A liquid crystal layer 90 is interposed between the lower and upper substrates 60 and 70. The pixel region P is defined with respect to the lower substrate 60 and an aperture PP that is substantially the display area is defined with respect to the lower and upper substrates 60 and 70.
On the lower substrate 60, a plurality of common electrodes 44 are disposed in the pixel region P on a transparent substrate 1. A gate insulation layer 45 is formed on the transparent substrate 1 while covering the plurality of common electrodes 44. The data line 52 is disposed on the gate insulation layer 45 between the common electrode 44 of one pixel region and the common electrode 44 of the other pixel region. A plurality of pixel electrodes 56 are formed on the gate insulation layer 45, and each pixel electrode 56 is disposed between two common electrodes 44. The common electrodes 44 and the pixel electrodes 56 are located in an alternate manner in the pixel region P. A passivation layer 57 is formed on the gate insulation layer 45 to cover the data line 52 and the plurality of pixel electrodes 56.
On the upper substrate 70, a black matrix 72 is disposed on the rear surface (the side facing the lower substrate) of a transparent substrate 1 in a position corresponding to the data line 52 and the adjacent common electrodes 44. A color filter layer 74 having red, green and blue colors is disposed on the rear surface of the transparent substrate 1 to cover portions of the black matrix 72. An overcoat layer 76 is disposed on the color filter layer 74. The overcoat layer 76 planarizes the rear surface of the upper substrate 70 and protects the liquid crystal layer 90 from the dye or pigment included in the color filter layer 74.
Lower and upper orientation films 58 and 77 are disposed on the inner surfaces of the lower and upper substrates 60 and 70, respectively, in order to align the liquid crystals of the liquid crystal layer 90. Thus, the lower orientation film 58 is between the passivation layer 57 and the liquid crystal layer 90, and the upper orientation film 77 is between the overcoat layer 76 and the liquid crystal layer 90. Furthermore, lower and upper polarizers 59 and 78 are formed on the outer surfaces of the lower and upper substrates 60 and 70, respectively.
However, the related art IPS-LCD device shown in FIGS. 3 and 4 has some problems. Since the common electrode 44 disposed in the center of the pixel region P is made of the opaque metal, the aperture ratio is low. Further, since the capacitor electrode and the portion of the common line only form the storage capacitor, the capacitance of the storage capacitor is too low to be enough.